The invention relates to a process for preparing alkyl mercaptan by reacting a reactant mixture comprising dialkyl sulphides and/or dialkyl polysulphides and optionally dialkyl ethers with hydrogen sulphide over heterogeneous catalysts.
Methyl mercaptan is an industrially important intermediate for the synthesis of methionine and for the preparation of dimethyl sulphoxide and dimethyl sulphone. Methyl mercaptan is prepared predominantly from methanol and hydrogen sulphide by reaction over a catalyst consisting of an aluminium oxide support and transition metal oxides and basic promoters.
In the reaction of methanol with hydrogen sulphide, at the typical reaction temperatures and using an economically viable hydrogen sulphide excess, the reaction equilibrium is such that dimethyl sulphide is always formed as well as methyl mercaptan. In addition to thioether formation, the reaction to give polysulphides (e.g., dimethyl disulphide) is also observed. These compounds are removed in the course of workup of the product gas stream. When no further economically viable utilization of these components is possible, the by-products are typically disposed of, for example by incineration or reaction with alkalis. This procedure lowers the overall selectivity of the preparation process for methyl mercaptan and hence the economic viability of the process. In this context, one alternative is the recycling of the sulphides or polysulphides into the process. When the sulphide level, according to U.S. Pat. No. 2,816,146, is kept sufficiently high by a recycling, the new formation of mercaptans from alcohols or ethers is suppressed. The process has the serious disadvantage that large amounts of sulphides have to be separated, condensed and, on recycling into the circuit, evaporated again. For this purpose, large amounts of heat and cooling energies are required.
Typical catalysts which are used in industrial processes for producing methyl mercaptan from methanol and hydrogen sulphide exhibit high selectivities for the formation of methyl mercaptan and lead to a comparatively low evolution of dimethyl sulphide and dimethyl disulphide. A problem in this connection is that these compounds accumulate in the circuit when they are recycled into the process, since the catalysts used in the prior art can only poorly establish the equilibrium between methyl mercaptan and dimethyl sulphide. This means that, in each case, at most a quarter of the undesired newly formed sulphide is converted in the case of recycling into the circuit.
As shown by DE-C 1193038, it is also possible to separate the sulphide and to convert it in a separate reaction step over a different catalyst to methyl mercaptan. However, it is necessary to select a high excess of hydrogen sulphide in order to achieve technically relevant conversions of dimethyl sulphide. DE-C 1193038 describes a process in which the sulphide formed, in an upstream reactor, together with the total amount of hydrogen sulphide required, is passed over a catalyst which efficiently establishes the reaction equilibrium between sulphide and mercaptan (precatalyst, e.g. MoO3/Al2O3). The reaction products obtained are subsequently, after addition of methanol or dimethyl ether, conducted over a main catalyst (K2WO4/Al2O3) over which the alcohol or the ether reacts with as yet unconverted hydrogen sulphide to give methyl mercaptan.
As described in the above-mentioned patent application, the separation of reaction product and hydrogen sulphide in the case of use of large hydrogen sulphide excesses is, however, found to be difficult.
JP 58159456 relates to a methyl mercaptan process in which the hydrogen sulphide conducted in the circuit is mixed with fresh hydrogen sulphide, and the overall H2S stream is subsequently divided between a methyl mercaptan reactor and a DMS cleavage reactor. Upstream of the methyl mercaptan reactor, one H2S substream is mixed with methanol, while the second substream passes into the cleavage reactor with the DMS. The product streams of the two reactors are subsequently fed together to a product workup.
U.S. Pat. No. 2,831,031 discloses catalysts based on pyrophosphoric acid on titanium dioxide, over which dimethyl sulphide is converted to methyl mercaptan with a maximum selectivity of 97% at a conversion of 42%. U.S. Pat. No. 4,005,149 and JP 5246203 describe aluminum oxides doped with cobalt molybdate or tungsten sulphide, with which dimethyl sulphide conversions of, respectively, 41 and 88% and, respectively, selectivities of 92 and 93% for methyl mercaptan can be achieved. Further catalysts claimed in U.S. Pat. No. 4,313,006 are zeolites (X, Y, L) doped with sodium or potassium ions, with which maximum methyl mercaptan selectivities of 65% are achieved with a dimethyl sulphide conversion of 70%. JP 58159456 relates to aluminium oxides modified with phosphorus oxides and tungsten oxides, with which a maximum DMS cleavage conversion of 40% can be achieved. The H2S/DMS ratio in the reactant gas is 2 to 28 in the above mentioned applications. Preference is given to pursuing a high H2S/DMS ratio in order to achieve sufficiently high DMS cleavage conversions. U.S. Pat. No. 4,005,149 describes a process for catalytically cleaving organic sulphides with hydrogen sulphide in the presence of sulphactive catalysts. As a result of the addition of carbon disulphide to the reaction mixture, the overall conversion of sulphides to mercaptans can be increased. A disadvantage of this process is the use of toxic carbon disulphide in the process, which has to be separated again from the reaction products in a costly manner. Generally, in the cleavage of dialkyl sulphides to mercaptans with hydrogen sulphide, high selectivities for mercaptan and the maximum suppression of by-products are pursued. In contrast, the decomposition of (poly)sulphides to mercaptans, for example over aluminium oxides, without the addition of hydrogen sulphide, is characterized by comparatively low selectivities for methyl mercaptan and a broad spectrum of by-products. Mashkina et al., describe, for example, in React. Kinet. Catal. Lett., Vol. 70, No. 1, 183-189, 2000, the decomposition of dimethyl disulphide to methyl mercaptan without H2S addition over acidic catalysts with maximum methyl mercaptan selectivities of 87%.
According to Koshelev, et al. [React. Kinet. Catal., Vol. 27, No. 2, 387-391 (1985)] for the cleavage of dimethyl sulphide with hydrogen sulphide over γ-aluminium oxide, a maximum activity is achieved when the catalysts have a large number of aprotic Lewis acid sites and basic sites of moderate strength. The catalysts based on 3.5% Na2O/Al2O3 described by Koshelev, et al. exhibit, at a DMS conversion of 9.5%, however, only maximum methyl mercaptan selectivities of 82%, while methyl mercaptan selectivities of 97% with a conversion of 38% are achieved over pure γ-Al2O3.